1. Field of the Invention
This invention relates to a rolling mill and a rolling method for rolling a strip material or a bar material, which passes through upper and lower rolling rolls, to a predetermined thickness.
2. Description of Related Art
FIG. 10 schematically shows a conventional four high cross rolling mill.
In the conventional four high cross rolling mill, as shown in FIG. 10, upper and lower work roll chocks 002 and 003 are supported inside a housing 001. Shaft portions of upper and lower work rolls 004 and 005 are rotatably supported by the upper and lower work roll chocks 002 and 003, respectively, and the upper work roll 004 and the lower work roll 005 are disposed so as to be opposed to each other. Upper and lower backup roll chocks 006 and 007 are supported above and below the upper and lower work roll chocks 002 and 003. Shaft portions of upper and lower backup rolls 008 and 009 are rotatably supported by the upper and lower backup roll chocks 006 and 007, respectively. The upper backup roll 008 and the upper work roll 004 are opposed to each other, while the lower backup roll 009 and the lower work roll 005 are opposed to each other. A screw down device 010 for imposing a rolling load on the upper work roll 004 via the upper backup roll chock 006 and the upper backup roll 008 is provided in an upper portion of the housing 001.
Upper crossheads 011 and 012 for horizontally supporting the upper backup roll chock 006 and the upper work roll chock 002 are provided in the upper portion of the housing 001 and positioned on an entry side and a delivery side of the housing 001. The upper crossheads 011, 012 are horizontally movable by roll cross mechanisms 013, 014. Lower crossheads 015 and 016 for horizontally supporting the lower backup roll chock 007 and the lower work roll chock 003 are provided in a lower portion of the housing 001 and positioned on the entry side and the delivery side of the housing 001. The lower crossheads 015, 016 are horizontally movable by roll cross mechanisms 017, 018.
Thus, when rolling is performed, a strip S is fed from the entry side of the housing 001, and passed between the upper work roll 004 given a predetermined load by the screw down device 010 and the lower work roll 005, whereby the strip S is rolled. The rolled strip S is delivered from the delivery side and supplied to a subsequent step.
The roll cross mechanisms 013, 014, 017, 018 are actuated before or during rolling, whereby the upper chocks 002, 006 and the lower chocks 003, 007 are moved in directions different from each other via the crossheads 011, 012, 015, 016. As a result, the upper work roll 004 and upper backup roll 008, and the lower work roll 005 and lower backup roll 009 are turned in opposite directions about a roll center so that their rotation axes will cross each other and the angle of their crossed axes will be set at a predetermined angle. By so doing, the crown of the strip is controlled.
When screw down cylinders impose a rolling load on an upper work roll in an ordinary rolling mill, it is desired that the center of the screw down cylinder presses downward a proper position of an upper backup roll chock corresponding to the shaft center of an upper backup roll (upper work roll). With the conventional cross rolling mill described above, the roll cross mechanisms 013, 014, 017, 018 are actuated, whereby the upper work roll 004 and upper backup roll 008, and the lower work roll 005 and lower backup roll 009 are caused to cross at a predetermined angle in order to control the strip crown. By so doing, however, the center OA of the screw down device 010 and the shaft center OR of the upper backup roll 008 (upper work roll 004) are displaced from each other upstream or downstream in a transport direction (offset amount F). Thus, the screw down device 010 cannot press the proper position of the upper backup roll chock 006 corresponding to the shaft center OR of the upper backup roll 008.
If a pressing force acts on a position displaced from the proper position of the upper backup roll chock 006 corresponding to the shaft center OR of the upper backup roll 008 by the action of the screw down device 010, a tipping moment occurs in the upper backup roll chock 006. As a result, the upper work roll 004 cannot apply a proper rolling load to the strip S, so that stable rolling does not take place, decreasing the accuracy of rolling. Because of the tipping moment occurring in the upper backup roll chock 006, one-sided contact occurs between the screw down device 010 and the upper backup roll chock 006, causing partial wear to shorten the life of the screw down device 010.